A former hostage revealed the feared trio, based in IS stronghold Raqqa, are nicknamed “The Beatles” because they are so well known in jihadist circles.

John was said to be the main rebel negotiator during talks earlier this year to release 11 IS hostages who were later released after ransom demands were met.

Language expert Professor Paul Kerswill, from the University of York, said he believed the man spoke in “multicultural London English”.

Prof Kerswill said: “He probably has a foreign language background but it sounds like multicultural London English, which is people from all kinds of backgrounds who mix in the East End, a new kind of cockney.”

“John” became the most wanted man in the world yesterday.

SAS troops, along with the UK’s intelligence services, MI5 and MI6, joined forces with America’s security agencies to launch a manhunt for the gloating killer.

They were joined by Brit spy centre GCHQ as they scoured every detail of the sick execution film.

Up to 500 Brits are thought to have joined the terror group.

Last night several UK-raised jihadists went online to threaten more atrocities against the West.

Khadijah Dare, from south London, who now lives in Syria, even boasted she would become the first British female to behead a Brit or US target.

The extremist, who once showed off pictures of her four-year-old son holding a gun, tweeted: “UK must b shaking up haha.”

PROTECTION: Barack Obama has vowed to do whatever it takes to protect American lives [AFP/GETTY]

David Cameron called off his holiday in Cornwall and returned to Downing Street for crisis meetings.

He said the beheading was “shocking and depraved”.

The PM added: “Let me condemn the barbaric and brutal act that has taken place and let’s be clear what this act is, it is an act of murder, and murder without any justification.

Foreign Secretary Philip Hammond said the UK would oppose IS “with every breath in our body”.

He added: “Our intelligence services will be looking very carefully on both sides of the Atlantic at this video to establish its authenticity, to try to identify the individual concerned and then we will work together to try to locate him.”

The killer warned President Obama that American air strikes against IS had to stop or a second hostage named as US journalist Steven Joel Soltoff would be next.

At the end of the beheading video, Mr Soltoff was held by the scruff of his neck in an orange jump suit. Experts warned British jihadists are among the most aggressive insurgents in the Islamic State’s ranks in Syria and Iraq.

Erin Saltman, of counterterrorism think-tank the Quilliam Foundation, said a Brit killer was chosen so would-be extremists could relate to the voice.

She said: “As soon as you have a fighter with a Middle East accent it becomes very easy to disassociate with that and say they’re brutal, they’re barbaric. But when you have a British citizen, raised in the UK, this is somebody we can empathise with.”